In long distance fiber optic communication systems it may be important to monitor the health of the system. For example, monitoring can be used to detect faults or breaks in the fiber optic cable, faulty repeaters or amplifiers or other problems with the system.
Known monitoring techniques include use of line monitoring equipment that generates a test signal representing a pseudo random bit sequence. The line monitoring equipment may transmit the test signal with the information signals, e.g. in wavelength division multiplexed system. The test signal may be returned to the line monitoring equipment through a high-loss loopback path within an amplifier or repeater. The line monitoring equipment may then separate the returned test signal from the data signals, and process the test signal to examine the health of the transmission system.